Analysis of wafer flatness for CD control in photolithography

Wafer-induced focus error is investigated for analysis of our focus budget in photolithography. Using a newly developed wafer monitor, NIWF-300 (Nikon Corp.), we directly measure surface flatness of the wafer placed on wafer holder with vacuum chuck. Single site polished Si wafers were evaluated with NIWF-300 and a conventional flatness monitor. We also investigated the effect of wafer holder using a ring-shape wafer support and a pin-shape wafer support. As a result, we found wafer shape measured in a freestanding condition does not represent surface flatness of the wafer on a holder. The holder has an impact on the wafer surface. The increase of adsorption ratio between wafer and holder improves the surface flatness.